JP-A-2003-61089 discloses a bit rate converter which can determine the picture quality of a desired portion in a video image on the basis of auxiliary information associated with the video image, thereby preventing the deterioration of the picture quality in that portion of the image which the picture producer considers important.
JP-A-2003-199102 discloses a bit rate converter which can improve the discernibility of animation images by detecting an important area in an image and allocating a large amount of code to the important area.
In storing animation images in a storage medium or transmitting them through a network, such an encoding system as MPEG-2 (ISO/IEC 13818) is used in order to reduce data amount while maintaining the quality of animation intact. In general, the amount of code per frame is determined depending on the bit rate that represents data amount per unit time. The bit rates for animation images are determined depending on the capacity of transmission channel for broadcasting. Therefore, when animation image data having been encoded under the condition that they are transmitted over a certain transmission channel, are to be transmitted over another transmission channel having a narrower bandwidth, the original animation image data must be converted to other animation image data having lower bit rates.
However, reducing bit rate may lead to deteriorating picture quality. To prevent this adverse effect, a variety of techniques have been devised which prevent the information that the viewers consider important from disappearing when recorded animation image data are converted to ones having lower bit rates.
The techniques disclosed in JP-A-2003-61089 and JP-A-2003-199102 are kinds of methods that suppress the deterioration of picture quality in a picture area of animation depending on whether the picture area is considered important. These methods may be suitable for the case where information is to be conveyed to viewers by means of images. According to these methods, for each frame of animation, the picture quality in an area containing an important object is prevented from deteriorating by allowing the picture quality in an area not containing an important object to deteriorate. However, what is important for the viewers of animation varies with time. For example, since, in a soccer game, winning the game depends on each team's point score, the scene of goal scoring is one of the most attractive scenes in the soccer game broadcasting program. Accordingly, motion picture frames covering a period of time from a moment just before the goal scoring to a moment just after the goal scoring can be said to be of higher importance than motion picture frames not covering the scene of goal scoring. It is therefore expected that the deterioration of a picture quality of the scene which viewers consider important can be prevented, if it is possible to adjust the amount of code to be allocated to the time intervals along the reproduction time axis depending on the importance of scene.